1. Field of the Invention
The present invention relates to vertical cavity surface emitting lasers (VCSELs) and photodetectors, and more particularly to the application of such optoelectronic devices where they must operate independently but where it is also desirable to have a transmitter and a receiver closely-spaced.
2. Art Background
There are a number of data communications applications which make use of optoelectronic sending and receiving devices (i.e. light emitters and photodetectors). For fiber optic data communication applications requiring less than 200 MBits/sec., light emitting diodes (LEDs) are the light emitters of choice because they are relatively inexpensive to manufacture. For applications requiring higher speeds, lasers are used as the light emitters.
Until recently, most high speed data communications applications employed edge emitting lasers in a serial (single channel) format. With the advent of Vertical Cavity Surface Emitting Lasers (VCSELs), many such applications are now implemented using VCSEL arrays which can be interfaced to ferrules carrying multiple fibers to transmit several bits of data in parallel. At the receiving end, an array of photodetectors is coupled to the multiple fibers. It is the ability to manufacture VCSELs in arrays (an advantage of LEDs) combined with their high speed of operation (an advantage of lasers) which makes VCSELs desirable in such applications.
For high-speed serial duplex data communications applications, however, separately packaged light emitters (usually edge emitting lasers) and photodetectors are still employed. For long-haul applications (typically having distances greater than 1 kilometer), wavelength division multiplexing is often employed to transmit and receive data for a duplex channel over the same fiber. Because the primary cost of a long-haul duplex serial data channel resides in the fiber and its installation, complex beam-splitting techniques can be justified at the ends of the channel to separate the transmit and receive data streams from the single fiber.
For short-haul or xe2x80x9cpremisesxe2x80x9d applications, however, the cost of fiber and fiber installation is relatively less important than the cost of the many transmit and receive functions. Thus, it is the cost of the data transmit and receive components, and particularly the optoelectronic devices and their packaging, which drives cost considerations for short-haul applications. Typical short-haul implementations of a high-speed serial fiber optics data communications channel operating in full duplex still employ two multimode fibers; each one to connect an individually packaged transmitting light emitter to an individually packaged receiving photodetector. This is because the cost of complex beam-splitting components can not be justified.
FIGS. 1a and 1b illustrate the components comprising a typical implementation of a transmit or receive link for a short-haul high-speed duplex data communications application. FIG. 1a illustrates a fiber assembly 12. A round ferrule 26 houses an optical fiber 28 which is located precisely in the center of ferrule 26. A typical diameter for ferrule 26 is approximately 2.5 mm. Ferrule 26 comes with a latching mechanism 30 which is used to clamp and secure the ferrule to a barrel 32 of an optical sub-assembly 10 which is depicted in FIG. 1b. Barrel 32 houses optoelectronic device 14 typically in a TO can package 16 which is centrally located in the barrel as shown. Optoelectronic device 14 is typicaly located at an appropriate point within can 16 by a standoff 2. Driver or amplifier circuitry is coupled to optoelectronic device 14 through leads 22. A window 18 is provided in the top of the can package to allow transmitted light out or received light in, depending upon whether the optoelectronic device is a light emitter or a photodetector. The TO package is aligned with fiber 28 and epoxied using epoxy 24 to fix the position of the optoelectronic device with respect to the ferrule 26 and hence fiber 28. Optical elements such as lens 20 are typically provided to focus the light for optimal optical efficiency, particularly where the light emitter is an edge emitting laser. Barrel 32 is designed to mate with latching mechanism 30 of fiber assembly 12.
Both fiber assembly 12 and barrel 10 are precision manufactured for precise mating. Active alignment TO package 16 and optoelectronic device must be performed in the x, y and z axes. First, the optoelectronic device must be precisely aligned within the package 16. Second, the package 16 must be precisely aligned within barrel 10. Finally, optical element 20 must be precisely aligned with respect to its distance from the optoelectronic device 14 to achieve proper optical operation. Because a separate package is required for both the transmit side and the receive side of the duplex data channel, a total of twelve active alignments must be performed for each channel and each channel includes the cost of eight precision-manufactured coupling parts.
FIGS. 1c and 1d provide a schematic illustration of the fiber assembly 12 and optoelectronics subassembly 10 of FIGS. 1a and 1b respectively.
FIG. 2 illustrates a typical duplex serial data communications module 40, which has mounted to it an optical subassembly 52 containing a light emitting device 13 disposed in a TO can package 9 having a window 17, which is to be mated with an optical fiber assembly 46 and which is dedicated to data transmission. Module 40 also has an optical subassembly 50 mounted to it containing a photodetector 15 disposed in TO can package 11 and which is to be mated with optical fiber assembly 48 and dedicated to receiving data from a remote module not shown. Because of the differing optical requirements of the transmit and receive devices, the modules must often be mounted in a staggered fashion as shown. Moreover, the transmit devices are located at an optically appropriate point in their can packages by standoffs 4 and 6 respectively.
Because of the cost of the precision components and the large number of alignments required for implementing duplex serial modules 40, it is highly desirable to integrate the transmit and receive optoelectronic devices (i.e. light emitter and photodetector) into one package. The integration of the two devices into a single package is not, however, an easily achieved solution. The prior art implementations as illustrated in FIGS. 1a-d and 2 cannot be readily adapted to multifiber ferrules currently available for unidirectional data transmission using VCSEL arrays. These multifiber ferrules have fiber spacings which are typically about 250 microns and can be less. The diameter of the TO can package 14 commonly used in present implementations is itself 5600 microns in diameter. Thus, the standard ferrule and barrel would have to grow substantially in diameter to accommodate two fibers having the spacing dictated by the TO cans housing the optoelectronic devices.
Even if a substantially larger barrel could be created to integrate the light emitter and photodetector as commonly packaged to receive both a transmit and a receive fiber, it is not clear that the resulting package could provide the necessary separation of incoming and scattered outgoing light beams to prevent crosstalk between the transmit and receive signals (at least not without complex optics and possibly some form of isolation). Although solutions have been disclosed to stack a light emitter (typically an LED) on top of a photodetector to transmit and receive wavelength division multiplexed signals (the light emitter is transparent to the received wavelength), beam-splitting must still be employed at the opposite end.
Thus, there is room in the art for an improvement in the area of optoelectronic device fabrication which facilitates the integration of one or more pairs of transmit and receive devices for interface with a single ferrule carrying one or more pairs of fibers having spacings as little as 250 microns or less, to substantially reduce the cost and complexity of implementing high-speed serial duplex data communications channels.
It is therefore an objective of the present invention to provide a VCSEL device sufficiently close to a photodetector device to permit the use of commercially available multifiber ferrules having fibers spaced on the order of 750 microns to 250 microns or less.
It is a further objective of the present invention to provide the closely-spaced but independently operable optoelectronic devices by monolithically integrating the two devices on the same substrate.
It is still further an objective of the invention to provide a process by which multiple pairs of VCSELs and photodetectors can be arrayed on the same substrate.
It is still further an objective of the invention to provide a process by which the closely-spaced but independently operable optoelectronic devices can be packaged using known lead-frame or ceramic packaging technologies.
It is still further an objective of the invention to integrate any requisite optics with either the semiconductor manufacturing technology or the packaging technology.
It is still further an objective of the present invention to simplify significantly the alignment of the fibers to the closely-spaced optoelectronic devices by taking advantage of the photolithographic nature of monolithic semiconductor processing to precisely define the separation between the optoelectronic devices.
It is still further an objective of the invention to provide closely-spaced but independently operable VCSEL and photodetector pairs capable of near-field operation requiring no optics and which permit butt coupling between a package containing the optoelectronic devices and a flat faced multifiber ferrule.
In a first preferred embodiment of the invention, one or more VCSELs are formed using a known process for manufacturing such devices. The one or more VCSELs comprise an n-type GaAs substrate and a first mirror formed on the substrate, which is a well-known distributed Bragg reflector (DBR) a first spacer or cladding layer which is formed on top of mirror. This first mirror is also doped n-type. An active region is then formed on top of the first mirror, the active region comprising at least one quantum well layer or bulk layer. A second spacer or cladding layer is formed on the active region, with a second DBR being formed on the second spacer layer and doped to have p-type conductivity.
On top of the VCSEL layers is grown an etch-stop layer of AlGaAs having about 90% or greater Al content. An extended p-type layer of AlGaAs having more typical alloy proportions is then grown on top of the etch-stop layer. On top of this p-type layer is grown an intrinsic layer (i) which is undoped GaAs. On top of the intrinsic layer is grown an n-type region of AlGaAs. An etching process is then performed to etch away the extended p, i, and n layers where the one or more VCSELs are to be formed. The etching process uses the etch-stop layer to mark the end of the etching process so that the VCSEL area has exposed the top surface of an appropriately designed mirror. A proton implant region is created which separates the one or more VCSELs and the photodetectors formed by the unetched p, i, and n layers. Anode contacts are formed over the non-implanted p regions to form apertures for the VCSELs. A VCSEL cathode contact is formed on the substrate. Anode contacts are also formed on the p region of the p-i-n photodiode and a cathode contact is made to the n region of the p-i-n photodiode.
Thus, in this preferred embodiment, a VCSEL circuit can be isolated from a p-i-n photodiode using a proton implant isolation region which is commonly used to isolate VCSELs formed in arrays. The anode contacts to the p region of the p-i-n photodiode may be coupled to ground so that the VCSEL structure which lies underneath the p-i-n photodiode is never turned on and other bipolar parasitic effects are avoided. The width of the proton implant isolation region is typically between 50 and 100 microns. Thus, the VCSEL and the p-i-n photodiode can be separated by an accurately known distance, significantly less than 250 microns if desired. Moreover, the difference in thickness between the VCSEL and the p-i-n photodiode is small, thereby permitting near-field coupling of the optoelectronic devices to fibers.
One significant advantage of the first embodiment of the invention is that it requires very few additional steps to an otherwise typical VCSEL manufacturing process. A second advantage is that, when an anti-reflection coating of silicon nitride is applied to the photo-receiving n region of the photodiode, in conjunction with the p-type mirror which underlies the p-i-n photodiode, a high degree of efficiency is achieved. The to silicon nitride anti-reflection coating increases transmission of incoming light into the surface of the p-i-n photodiode. Additionally, any light which is not absorbed by the intrinsic layer of the photodiode on its way through will be reflected from the underlying p-type mirror back into the intrinsic layer and will then have a second opportunity to be absorbed.
A second preferred embodiment of the invention employs a VCSEL with an MSM photodiode. The VCSEL is manufactured on a semi-insulating substrate. Because the MSM photodetector employs the semi-insulating layer as its common cathode, the two optoelectronic devices are virtually isolated from one another electronically as a result. A photolithographically defined minimum spacing of 250 microns or less can also be achieved using the second preferred embodiment of the invention. An anti-reflection coating is also preferably employed over the MSM photodetector to increase efficiency. To further enhance electrical isolation between the two devices, an isolation region can also be formed, preferably by implantation. Another advantage of using an MSM photodiode is that the two anode terminals can be used to drive a differential amplifier, thereby permitting common-mode rejection of noise.
Either of the two preferred embodiments can be integrated with optically transmissive materials which can be formed into lenses on the surface of the semiconductor. Either embodiment can also be implemented within standard precision manufactured barrels to be aligned with circular ferrules containing multiple fibers. Finally, either embodiment can be encapsulated using known lead-frame or ceramic packaging technology to permit near-field flat coupling between a flat package having an optically transmissive surface and a commercially available flat rectangular ferrule containing multiple fibers.
The foregoing objectives and the features of the preferred embodiments will be understood by those of skilled in the art with reference to the following detailed description.